Golf ball

ABSTRACT

A golf ball which is formed from a vulcanized rubber compound composed of 100 parts by weight of a rubber base material, 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof, 0.1 to 5 parts by weight of an organosulfur compound, 5 to 80 parts by weight of an inorganic filler, and 0.1 to 3 parts by weight of an organic peroxide, the rubber base material being composed of a first polybutadiene accounting for 50 to 95 wt % and a second polybutadiene accounting for 5 to 50 wt %, both polybutadiene being synthesized by using a rare earth element-based catalyst, the first polybutadiene containing no less than 60 wt % of cis-1,4 bonds and having a Mooney viscosity (ML 1+4  (100° C.)) no lower than 50 and the second polybutadiene containing no less than 60 wt % of cis-1,4 bonds and having a Mooney viscosity (ML 1+4  (100° C.)) no higher than 45. The golf ball is characterized by good rebound resilience and good workability with which it is produced.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball characterized by goodrebound resilience, good flying performance, and good extrusionworkability with which it is produced. The golf ball has a solid coreformed from an improved rubber compound.

In an attempt to impart good rebound resilience to golf balls, variousimprovements have been made on the formulation of polybutadiene as abase material for golf balls.

Among such improvements is a new rubber compound for solid balls whichwas disclosed in Japanese Patent Laid-open No. Sho 62-89750, forexample. This rubber compound is composed of a polybutadiene (as thebase material) having a Mooney viscosity of 70 to 100, which issynthesized by using a nickel or cobalt catalyst, and anotherpolybutadiene having a Mooney viscosity of 30 to 90, which issynthesized by using a lanthanum-based catalyst, or anotherpolybutadiene having a Mooney viscosity of 20 to 50, which issynthesized by using a nickel or cobalt catalyst.

However, there still exists a demand for improvement on reboundresilience.

Also, there is disclosed in Japanese Patent Laid-open No. Hei 2-268778 agolf ball which is molded from a blend of a polybutadiene having aMooney viscosity less than 50, which is synthesized by using a groupVIII element catalyst, and another polybutadiene having a Mooneyviscosity less than 50, which is synthesized by using a lanthanidecatalyst. This golf ball, however, is poor in rebound resilience.

Moreover, there is disclosed in Japanese Patent Laid-open No. Hei11-70187 a multi-piece solid golf ball having an intermediate layerformed from a polybutadiene having a low Mooney viscosity. There isdisclosed in Japanese Patent Laid-open No. Hei 11-319148 a solid golfball formed from a rubber compound composed of a polybutadiene having aMooney viscosity of 50 to 69, which is synthesized by using a nickel orcobalt catalyst, and another polybutadiene having a Mooney viscosity of20 to 90, which is synthesized by using a lanthanoid-based catalyst.There is disclosed in Japanese Patent Laid-open No. Hei 11-164912 asolid golf ball formed from a rubber compound which contains no morethan 2.0% of 1,2-vinyl bond and has an Mw/Mn ratio no larger than 3.5(which is a ratio of weight-average molecular weight to number-averagemolecular weight). There is disclosed in Japanese Patent Laid-open No.Sho 63-275356 a golf ball which is formed from a rubber compoundcontaining a polybutadiene having a high Mooney viscosity. There isdisclosed in Japanese Patent Laid-open No. Hei 3-151985 a golf ballwhich is formed from a rubber compound composed of two kinds ofpolybutadiene, one having a high number-average molecular weight and theother having a low number-average molecular weight. However, all thedisclosed rubber compounds merely give golf balls poor in reboundresilience.

There is disclosed in Japanese Patent Laid-open No. Sho 61-71070 arubber compound with two kinds of organic peroxide, and there isdisclosed in Japanese Patent Laid-open No. Sho 62-112574 a rubbercompound with a smaller amount of organic peroxide. These rubbercompounds, however, are poor in rebound resilience and slow incrosslinking, which leads to extremely low productivity.

Also, the rubber compounds disclosed in Japanese Patent Laid-open Nos.2001-149505 to 2001-149507 still have room for improvement on reboundresilience.

SUMMARY OF THE INVENTION

The present invention was completed in view of the foregoing. It is anobject of the present invention to provide a golf ball characterized bygood shot feel, good rebound resilience, and good flying performance.The golf ball permits production with good extrusion workability, shortvulcanizing time, and high productivity.

In order to achieve the above-mentioned object, the present inventorscarried out extensive researches, which led to the following finding. Asolid golf ball with good rebound resilience can be produced with highproductivity if its solid core is formed from a vulcanized product of arubber compound which is composed of a rubber base material andadditives as specified below. The rubber base material is composed oftwo kinds of polybutadiene, both synthesized by using a rare earthelement-based catalyst. The two polybutadienes contain cis-1,4 bonds inan amount less than 60 wt %, but one has a Mooney viscosity (ML₁₊₄ (100°C.)) no lower than 50 and the other has a Mooney viscosity no higherthan 45. The additives include an unsaturated carboxylic acid and/or ametal salt thereof, an organosulfur compound, an inorganic filler, andan organic peroxide. Their amount based on 100 parts by weight of therubber base material is 10 to 60, 0.1 to 5, 5 to 80, and 0.1 to 3 partsby weight, respectively. The organic peroxide should be composed of twoor more species differing half-life such that the ratio of b_(t)/a_(t)is 7 to 20, where a_(t) denotes the half-life of an organic peroxide (a)having the shortest half-life at 155° C. and b_(t) denotes the half-lifeof an organic peroxide (b) having the longest half-life at 155° C.

In other words, it was found that a rubber compound exhibits goodextrusion workability and yields golf balls that excel in reboundresilience if it is composed of a high-Mooney viscosity polybutadieneand a low-Mooney viscosity polybutadiene, both synthesized by using arare earth metal catalyst. It was also found that, unlike theconventional technology in which a rubber compound with an organicperoxide in a reduced amount needs a long vulcanizing time (which leadsto low productivity) and has poor rebound resilience, a rubber compoundimproves in workability and decreases in vulcanizing time (therebyimproving in productivity) and exhibits higher rebound resilience, if itis based on a high-resilient polybutadiene synthesized by using a rareearth element-based catalyst and it is also compounded with a smallamount of two or more organic peroxides greatly differing in half-life.The present invention is based on the foregoing finding.

The present invention is directed to a golf ball defined in thefollowing.

-   [1] A golf ball which is formed from a vulcanized rubber compound    composed of 100 parts by weight of a rubber base material, 10 to 60    parts by weight of an unsaturated carboxylic acid and/or a metal    salt thereof, 0.1 to 5 parts by weight of an organosulfur compound,    5 to 80 parts by weight of an inorganic filler, and 0.1 to 3 parts    by weight of an organic peroxide, the rubber base material being    composed of a first polybutadiene accounting for 50 to 95 wt % and a    second polybutadiene accounting for 5 to 50 wt %, both polybutadiene    being synthesized by using a rare earth element-based catalyst, the    first polybutadiene containing no less than 60 wt % of cis-1,4 bonds    and having a Mooney viscosity (ML₁₊₄ (100° C.)) no lower than 50 and    the second polybutadiene containing no less than 60 wt % of cis-1,4    bonds and having a Mooney viscosity (ML₁₊₄ (100° C.)) no higher than    45.-   [2] A golf ball of [1], wherein the first polybutadiene has a    molecular weight distribution Mw/Mn of 2.0 to 6.0, where Mw denotes    a weight-average molecular weight and Mn denotes a number-average    molecular weight.-   [3] A golf ball of [1], wherein the first polybutadiene is a    modified polybutadiene rubber which is obtained by synthesizing an    ordinary polybutadiene with a Nd-based catalyst and subsequently    reacting the thus obtained polybutadiene with an end group modifier.-   [4] A golf ball of [1], wherein the second polybutadiene has a    molecular weight distribution Mw/Mn of 2.0 to 6.0, where Mw denotes    a weight-average molecular weight and Mn denotes a number-average    molecular weight.-   [5] A golf ball of [1], wherein the second polybutadiene is a    modified polybutadiene rubber which is obtained by synthesizing an    ordinary polybutadiene with a Nd-based catalyst and subsequently    reacting the thus obtained polybutadiene with an end group modifier.-   [6] A golf ball of [1], wherein the second polybutadiene has a    Mooney viscosity (ML₁₊₄ (100° C.)) no higher than 40.-   [7] A golf ball of [1], wherein the organic peroxide is composed of    two or more species of organic peroxides which are specified by    their half-life such that the ratio of b_(t)/a_(t) is 7 to 20, where    at denotes the half-life of an organic peroxide (a) having the    shortest half-life at 155° C. and b_(t) denotes the half-life of an    organic peroxide (b) having the longest half-life at 155° C.-   [8] A golf ball of [1], wherein the total content of the organic    peroxide is 0.1 to 0.8 part by weight for 100 parts by weight of the    rubber base material.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more detail in the following.

The golf ball according to the present invention is formed from aspecific vulcanized rubber compound which is composed of,

(A) a rubber base material composed mainly of two polybutadienessynthesized by using a rare earth element-based catalyst, bothcontaining no less than 60 wt % of cis-1,4 bonds and differing in Mooneyviscosity (M₁₊₄ (100° C.)) from each other,

(B) an unsaturated carboxylic acid and/or a metal salt thereof,

(C) an organosulfur compound,

(D) an inorganic filler, and

(E) an organic peroxide.

The first and second polybutadienes constituting component (A) shouldcontain cis-1,4 bonds in an amount no less than 60%, preferably no lessthan 80%, more preferably no less than 90%, and most desirably no lessthan 95%. (“%” means “wt %” hereinafter.) The rubber compound will bepoor in rebound resilience if the content of cis-1,4 bonds isexcessively small.

Moreover, the polybutadienes should contain 1,2-vinyl bonds in an amountno more than 2%, preferably no more than 1.7%, and more preferably nomore than 1.5%. The rubber compound will be poor in rebound resilienceif the content of 1,2-vinyl bonds is excessively large.

The first polybutadiene should have a Mooney viscosity (ML₁₊₄ (100° C.))no smaller than 50, preferably no smaller than 51, more preferably nosmaller than 52, and most desirably no smaller than 54, and no largerthan 140, preferably no larger than 120, more preferably no larger than100, and most desirably no larger than 80.

The second polybutadiene should have a Mooney viscosity no large than45, preferably no larger than 40, more preferably no larger than 38, andmost desirably no larger than 36, and no smaller than 10, preferably nosmaller than 15, more preferably no smaller than 20, and most desirablyno smaller than 25.

Incidentally, “Mooney viscosity” used in the present invention is anindex of viscosity for industrial use which is measured (according toJIS K6300) with a Mooney viscometer as a rotary plastometer. It isexpressed in terms of ML₁₊₄ (100° C.), where M stands for Mooneyviscosity, L stands for large rotor (L type), 1+4 indicates thatduration of preheating is one minute and duration of rotor rotation isfour minutes, and 100° C. is the temperature at which measurement iscarried out.

The polybutadiene used in the present invention should be one which issynthesized by using a rare earth element-based catalyst selected fromany known ones.

An example of the catalyst is a combination of a compound of a lanthanumrare earth element with an organoaluminum compound, alumoxane, halogencontaining compound, and Lewis base (optional).

The compound of a lanthanum rare earth element includes halides,carboxylates, alcoholates, thioalcoholates, and amides of metals with anatomic number 57 to 71.

The organoaluminum compound includes those which are represented byAlR¹R²R³ (where R¹, R², and R³ are identical or different groups, eachdenoting hydrogen or a residue of C₁₋₈ hydrocarbon compound).

The almoxane is a compound having the structure represented by theformula (I) or (II) below. It may take on an associated form asdescribed in Fine Chemical 23, (9), 5 (1994), J. Am. Chem. Soc., 115,4971 (1993), and J. Am. Chem. Soc., 117, 6465 (1995).

(where, R⁴ is a C₁₋₂₀ hydrocarbon group and n is an integer of 2 orabove.)

The halogen containing compound may be any of aluminum halide, strontiumhalide, and metal halides. The aluminum halide is a compound representedby AlX_(n)R_(3-n) (where, X denotes a halogen, R denotes a residue ofC₁₋₂₀ hydrocarbon compound, such as alkyl, aryl, and aralkyl, and n is1, 1.5, 2, or 3). The strontium halide includes Me₃SrCl, Me₂SrCl₂,MeSrHCl₂, and MeSrCl₃. The metal halide includes silicon tetrachloride,tin tetrachloride, and titanium tetrachloride.

The Lewis base includes acetylacetone and ketone alcohol, which are usedto complex the compound of lanthanoid series rare earth element.

The lanthanum series rare earth compound which is advantageously used asa catalyst in the present invention is a neodymium compound. It ishighly active to yield a polybutadiene with a high content of 1,4-cisbonds and a low content of 1,2-vinyl bonds. Examples of this catalystare given in Japanese Patent Laid-open No. Hei 11-35633.

For the polybutadiene polymerized by using a rare earth element-basedcatalyst (or lanthanum series rare earth compound) to have the ciscontent and Mw/Mn ratio as specified above, it is desirable that theamount of butadiene should be 1,000 to 2,000,000 times, particularly5,000 to 1,000,000 times, the amount of the lanthanum series rare earthcompound (in molar ratio). It is also desirable that the molar ratio ofAlR¹R²R³ to the lanthanum series rare earth compound should be from 1 to1,000, particularly from 3 to 500. It is also desirable that the molarratio of the halogen compound to the lanthanum series rare earthcompound should be from 0.1 to 30, particularly from 0.2 to 15. It isalso desirable that the molar ratio of the Lewis base to the lanthanumseries rare earth compound should be from 0 to 30, particularly from 1to 10. Polymerization may be bulk polymerization or vapor phasepolymerization with or without solvent. The polymerization temperatureis usually from −30° C. to 150° C., preferably from 10° C. to 100° C.

Polymerization of butadiene by using the rare earth element-basedcatalyst may be either bulk polymerization or vapor phase polymerizationwith or without solvent. The polymerization temperature is usually from−30° C. to 150° C., preferably from 10° C. to 100° C.

The polybutadiene as component (A) in the present invention may be amodified polybutadiene which is obtained if polymerization by using arare earth element-based catalyst is followed by reaction of thepolymer's active end groups with an end group modifier.

The modified polybutadiene rubber may be obtained by using any of thefollowing seven end group modifiers.

-   (1) A compound having one or more alkoxysilyl groups, or desirably    an alkoxysilane compound having at least one epoxy group or    isocyanate group in the molecule. Examples of epoxy group containing    alkoxysilane are listed below.-   3-glycidyloxypropyltrimethoxysilane,-   3-glycidyloxypropyltriethoxysilane,-   (3-glycidyloxypropyl)methyldimethoxysilane,-   (3-glycidyloxypropyl)methyldiethoxylsilane,-   β-(3,4-epoxycyclohexyl)trimethoxysilane,-   β-(3,4-epoxycyclohexyl)triethoxysilane,-   β-(3,4-epoxycyclohexyl)methyldimethoxysilane,-   β-(3,4-epoxycyclohexyl)ethyldimethoxysilane, condensate of    3-glycidyloxypropyltrimethoxysilane, and condensate of    (3-glycidyloxypropyl)methyldimethoxysilane.-   Examples of isocyanate group containing alkoxysilane are listed    below.-   3-isocyanatepropyltrimethoxysilane,-   3-isocyanatepropyltriethoxysilane,-   (3-isocyanatepropyl)methyldimethoxysilane,-   (3-isocyanatepropyl)methyldiethoxysilane, condensate of    3-isocyanatepropyltrimethoxysilane, and condensate of    (3-isocyanatepropyl)methyldimethoxysilane.

Any of the alkoxysilane compounds listed above may be used incombination with a Lewis acid to enhance its reactivity when it isapplied to active end groups. The Lewis acid functions as a catalyst topromote the coupling reaction, so that the resulting modified polymerhas improved storage stability with less cold flow. Examples of theLewis acid include dialkyltin dialkyl malate, dialkyltin dicarboxylate,and aluminum trialkoxide.

-   (2) Any of halogenated organometallic compounds, metal halide    compounds, and organometallic compounds, which are represented by    the following formulas.    R⁵ _(n)M′X_(4−n), M′X₄, M′X₃, R⁵ _(n)M′(—R⁶—COOR⁷)_(4−n), and R⁵    _(n)M′(—R⁶—COR⁷)_(4−n)    (wherein R⁵ and R⁶ are identical or different C₁₋₂₀ hydrocarbon    group; R⁷ is a C₁₋₂₀ hydrocarbon group which may have a carbonyl or    ester group in the side chain; M′ is a tin atom, silicon atom,    germanium atom, or phosphorus atom; X is a halogen atom; and n is an    integer of 0 to 3.)-   (3) Heterocumulene compound having in the molecule a Y═C═Z bond    (where Y is a carbon atom, oxygen atom, nitrogen atom, or sulfur    atom; and Z is an oxygen atom, nitrogen atom, or sulfur atom).-   (4) Three-membered heterocyclic compound having in the molecule the    following bonds.

(where Y is an oxygen atom, nitrogen atom, or sulfur atom).

-   (5) Halogenated isocyano compounds.-   (6) Any of carboxylic acids, acid halides, ester compounds,    carboxylic ester compounds, and acid anhydrides, which are    represented by the following formulas.    R⁸—(COOH)_(m), R⁹(COX)_(m), R¹⁰—(COO—R¹¹)_(m), R¹²—OCOO—R¹³,    R¹⁴—(COOCO—R¹⁵)_(m), and

(where, R⁸ to R¹⁶ are identical or different C₁₋₅₀ hydrocarbon groups, Xis a halogen atom, and m is an integer of 1 to 5.)

-   (7) Metal carboxylate represented by any of the following formulas.    R¹⁷ ₁M″(OCOR¹⁸)⁴⁻¹, R₁₉ ₁M″(OCO—R²⁰—COOR²¹)⁴⁻¹, and

(where, R¹⁷ to R²³ are identical or different C₁₋₂₀ hydrocarbon group;M″ is a tin atom, silicon atom, or germanium atom; and 1 is an integerof 0 to 3).

Typical examples and usage of the end group modifiers listed above areshown in Japanese Patent Laid-open Nos. Hei 11-35633, Hei 7-268132, and2002-29399.

According to the present invention, the-first and second polybutadienesshould have a molecular weight distribution Mw/Mn (where Mw denotes aweight-average molecular weight and Mn denotes a number-averagemolecular weight), which is no smaller than 2.0, preferably no smallerthan 2.2, more preferably no smaller than 2.4, most desirably no smallerthan 2.6, and no larger than 6.0, preferably no larger than 5.0, morepreferably no larger than 4.0, most desirably no larger than 3.4. Withan excessively small or larger value of Mw/Mn, the rubber compound willbe poor in workability or poor in rebound resilience, respectively.

According to the present invention, the rubber base material is composedmainly of the first and second polybutadienes. The amount of the firstpolybutadiene in the rubber base material should be no less than 50%,preferably no less than 60%, more preferably no less than 70%, mostdesirably no less than 75%, and no more than 95%, preferably no morethan 90%, more preferably no more than 85%, most desirably no more than80%. On the other hand, the amount of the second polybutadiene in therubber base material should be no less than 5%, preferably no less than10%, more preferably no less than 15%, most desirably no less than 20%,and no more than 50%, preferably no more than 40%, more preferably nomore than 30%, most desirably no more than 20%. The first polybutadienein an excessively large amount will hamper extrusion workability, andthe second polybutadiene in an excessively large amount will adverselyaffect rebound resilience.

Incidentally, the rubber base material may optionally be compounded withother rubber components than the polybutadienes. Such rubber componentsinclude polybutadiene excluding the one mentioned above polymerized byusing a catalyst which is a metal compound of a group VIII element,diene rubber (such as styrene butadiene rubber), natural rubber,isoprene rubber, and ethylene propylene diene rubber.

The unsaturated carboxylic acids used as component (B) include acrylicacid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid andmethacrylic acid are especially preferred. Metal salts of theunsaturated carboxylic acid include zinc salt and magnesium salt. Zincacrylate is especially preferred.

The amount of the unsaturated carboxylic acid and/or salt thereof ascomponent (B), based on 100 parts of the rubber base material, should beno less than 10 parts, preferably no less than 15 parts, more preferablyno less than 20 parts, and no more than 60 parts, preferably no morethan 50 parts, more preferably no more than 45 parts, most desirably nomore than 40 parts. (“Parts” means “parts by weight” hereinafter.)

The organosulfur compound as component (C) includes thiophenols,thionaphthols, halogenated thiophenols and metal salts thereof, andpolysulfides having 2 to 4 sulfur atoms. Typical examples includepentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol,p-chlorothiophenol and zinc salts thereof, alkylphenyl polysulfideshaving 2 to 4 sulfur atoms (such as diphenylpolysulfide,dibenzylpolysulfide, dibenzoylpolysulfide, dibenzothiazoylpolysulfide,and dithiobenzoylpolysulfide), alkylphenyldisulfide, sulfur compoundhaving a furan ring, and sulfur compounds having a thiophen ring. Ofthese examples, diphenyldisulfide and zinc salt of pentachlorothiophenolare especially preferred.

The amount of the organosulfur compound, based on 100 parts of therubber base material as component (A), should be no less than 0.1 part,preferably no less than 0.2 part, more preferably no less than 0.4 part,most desirably no less than 0.7 part, and no more than 5 parts,preferably no more than 4 parts, more preferably no more than 3 parts,most desirably no more than 2 parts, particularly no more than 1.5. Withan excessively small amount, the organosulfur compound does not producethe effect of improving rebound resilience; the rubber compoundcontaining the organosulfur compound in an excessively large amount istoo soft to provide sufficient rebound resilience.

The inorganic filler as component (D) includes zinc oxide, bariumsulfate, and calcium carbonate. The amount of the inorganic filler for100 parts of component (A) should be no less than 5 parts, preferably noless than 7 parts, more preferably no less than 10 parts, most desirablyno less than 13 parts, and no more than 80 parts, preferably no morethan 65 parts, more preferably no more than 50 parts, most desirably nomore than 40 parts. With an excessively large or small amount, theinorganic filler does not provide an adequate weight and good reboundresilience.

The organic peroxide as component (E) may be selected from known ones.Its typical examples include dicumyl peroxide,1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, andα,α′-bis(t-butylperoxy)diisopropylbenzene. They are commerciallyavailable under such trade names as Percumyl D (from NOF Corporation),Perhexa 3M (from NOF Corporation), and Luperco 231XL (from Atochem Co.).

The organic peroxide may be one species or a combination of two or morespecies. It is desirable to use two or more species of organic peroxidesfrom the standpoint of rebound resilience. In this case, they shoulddiffer in half-life as specified below. Assuming that the half-life at155° C. of one organic peroxide having the shortest half-life isdesignated as a, and the half-life at 155° C. of another organicperoxide having the longest half-life is designated as b_(t), then theratio of b_(t)/a_(t) should be no smaller than 7, preferably no smallerthan 8, more preferably no smaller than 9, most desirably no smallerthan 10, and no larger than 20, preferably no larger than 18, morepreferably no larger than 16, most preferably no larger than 14. Therubber compound failing to meet this requirement will be poor in reboundresilience, compression, and durability.

The organic peroxide (a) should have a half-life at (at 155° C.) whichis no shorter than 5 seconds, preferably no shorter than 10 seconds,more preferably no shorter than 15 seconds, and no longer than 120seconds, preferably no longer than 90 seconds, more preferably no longerthan 60 seconds. The organic peroxide (b) should have a half-life b_(t)(at 155° C.) which is no shorter than 300 seconds, preferably no shorterthan 360 seconds, more preferably no shorter than 420 seconds, and nolonger than 800 seconds, preferably no longer than 700 seconds, morepreferably no longer than 600 seconds. The organic peroxide (a) shouldpreferably be 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane, and theorganic peroxide (b) should preferably be dicumyl peroxide.

The total amount of the organic peroxides, based on 100 parts of therubber base material as component (A), should be no less than 0.1 part,preferably no less than 0.2 part, more preferably no less than 0.3 part,and most desirably no less than 0.4 part, and no more than 3 parts,preferably no more than 2 parts, more preferably no more than 1 part,most desirably no more than 0.8 part, particularly no more than 0.6part. With an excessively small amount, the rubber compound takes a longtime for crosslinking (which leads to low productivity) and thecrosslinked product is subject to large compression. With an excessivelylarge amount, the crosslinked product is poor in rebound resilience anddurability.

In the case where two or more species of organic peroxides (a) and (b)are used in combination, the amount of (a), based on 100 parts ofcomponent (A), should be no less than 0.05 part, preferably no less than0.08 part, more preferably no less than 0.1 part, and no more than 1.5parts, preferably no more than 1 part, more preferably no more than 0.7part, most desirably no more than 0.4 part; and the amount of (b), basedon 100 parts of component (A), should be no less than 0.05 part,preferably no less than 0.15 part, more preferably no less than 0.2part, and no more than 1.5 parts, preferably no more than 1 part, morepreferably no more than 0.7 part, most desirably no more than 0.4 part.

The rubber compound may optionally be compounded with an antioxidant.The amount of the antioxidant for 100 parts of component (A) should beno less than 0.05 part, preferably no less than 0.1 part, morepreferably no less than 0.2 part, and no more than 3 parts, preferablyno more than 2 parts, more preferably no more than 1 part, mostdesirably no more than 0.5 part. The antioxidant may be selected fromcommercial ones, such as Nocrac NS-6, Nocrac NS-30 (both from OuchiShinko Chemical Industry Co., Ltd.), and Yoshinox 425 (from YoshitomiPharmaceutical Industries, Ltd.).

The rubber compound mentioned above may be vulcanized and cured in thesame way as the known rubber compound for golf balls. Vulcanization willtake 10 to 40 minutes at 100 to 200° C.

The vulcanized product mentioned above may be formed into golf balls ofvarious types in any manner such that they are uniform or varied inhardness across their cross section.

The golf ball according to the present invention may vary in types, suchas one-piece golf ball or multi-piece golf ball having a solid core or asolid center. The one-piece golf ball, solid core, or solid centershould have adequate stiffness so that their deflection under a load of980 N (100 kg) is usually no less than 2.0 mm, preferably no less than2.5 mm, more preferably no less than 2.8 mm, most desirably no less than3.2 mm, and no more than 6.0 mm, preferably no more than 5.5 mm, morepreferably no more than 5.0 mm, most desirably no more than 4.5 mm. Anexcessively large deflection leads to an unpleasant shot feel and ashort flying distance due to deformation and extreme spin caused by adriver shot. An excessively small deflection not only leads to a dullshot feel and an insufficient rebound resilience (which in turn leads toa short flying distance) but also adversely effects resistance tocracking caused by repeated shots.

The golf ball according to the present invention is made up of thevulcanized product obtained from the rubber compound mentioned above.The type of the golf ball is not specifically restricted. It may be aone-piece golf ball, which is formed entirely from the vulcanizedproduct. It may be a two-piece solid golf ball consisting of a solidcore and a cover layer formed thereon. The solid core is formed from thevulcanized product. It may be a multi-piece solid golf ball (orthree-piece golf ball) consisting of a solid core and two or more coverlayers. The solid core is formed from the vulcanized product. It may bea wound golf ball having a center core which is formed from thevulcanized product. Two-piece and multi-piece solid golf balls, in whichthe solid core is formed from the vulcanized product of the presentinvention, are desirable from the standpoint of good extrudability, goodvulcanizability, and good rebound resilience.

The solid core should have a diameter no smaller than 30.0 mm,preferably no smaller than 32.0 mm, more preferably no smaller than 35.0mm, most desirably no smaller than 37.0 mm, and no larger than 41.0 mm,preferably no larger than 40.5 mm, more preferably no larger than 40.0mm, most desirably no larger than 39.5 mm. Particularly, the solid corefor two-piece solid golf balls should have a diameter no smaller than37.0 mm, preferably no smaller than 37.5 mm, more preferably no smallerthan 38.0 mm, most desirably no smaller than 38.5 mm, and no larger than41.0 mm, preferably no larger than 40.5 mm, more preferably no largerthan 40.0 mm. The solid core for three-piece solid golf balls shouldhave a diameter no smaller than 30.0 mm, preferably no smaller than 32.0mm, more preferably no smaller than 34.0 mm, most desirably no smallerthan 35.0 mm, and no larger than 40.0 mm, preferably no larger than 39.5mm, more preferably no larger than 39.0 mm.

The solid core should have a specific gravity no lower than 0.9,preferably no lower than 1.0, more preferably no lower than 1.1, and nohigher than 1.4, preferably no higher than 1.3, more preferably nohigher than 1.2.

In the case where the present invention is applied to two-piece solidgolf balls or multi-piece golf balls, the solid core is made from thevulcanized product mentioned above and the solid core is enclosed withany known cover material and intermediate layer material by injectionmolding or pressure molding.

The cover material or intermediate layer material should be based on apolyurethane elastomer (thermoplastic or thermosetting), polyesterelastomer, ionomer resin, or polyolefin elastomer, or a mixture thereof.They may be used alone or in combination with one another. Of thesematerials, thermoplastic polyurethane elastomers and ionomer resins arepreferable.

The thermoplastic polyurethane elastomer mentioned above is commerciallyavailable as exemplified below. Pandex T7298, Pandex T7295, PandexT7890, Pandex TR3080, Pandex T8295, Pandex T8290, and Pandex T8260 (allfrom DIC Bayer Polymer, Ltd.), which are produced from an aliphatic oraromatic diisocyanate. The ionomer resin mentioned above is commerciallyavailable as exemplified below. Surlyn 6320, Surlyn 8120 and Surlyn 9945(both from E.I. du Pont de Nemours and Co., Inc.), and Himilan 1706,Himilan 1605, Himilan 1855, Himilan 1601, Himilan 1557 (all fromMitsui-DuPont Polychemicals Co., Ltd.).

In addition, the cover material or intermediate layer material mayoptionally be compounded with a thermoplastic elastomer or polymer(excluding those mentioned above), such as polyamide elastomer, styreneblock elastomer, hydrogenated polybutadiene, and ethylene-vinyl acetate(EVA) copolymer.

The two-piece solid golf ball or multi-piece solid golf ball accordingto the present invention may be produced by any known method withoutspecific restrictions. A preferred method for two-piece or multi-piecegolf balls consists of placing the solid core (formed from thevulcanized product mentioned above) in the injection mold and formingthe cover layer (in the case of two-piece golf ball) or forming theintermediate layers and cover layer (in the case of multi-piece golfball) by injection molding. In some cases, the cover layer may be formedby pressure molding.

The intermediate layer of the multi-piece solid golf ball should have athickness no smaller than 0.5 mm, preferably no smaller than 1.0 mm, andno larger than 3.0 mm, preferably no larger than 2.5 mm, more preferablyno larger than 2.0 mm, most desirably no larger than 1.6 mm.

The cover layer of both the two-piece solid golf ball and themulti-piece solid golf ball should have a thickness no smaller than 0.7mm, preferably no smaller than 1.0 mm, and no larger than 3.0 mm,preferably no larger than 2.5 mm, more preferably no larger than 2.0 mm,most desirably no larger than 1.6 mm.

The golf ball according to the present invention should conform to therules for competition, which state that the diameter should be nosmaller than 42.67 mm and the weight should be no more than 45.93 g. Theupper limit of the diameter should preferably be no larger than 44.0 mm,more preferably no larger than 43.5 mm, most desirably no larger than43.0 mm. The lower limit of the weight should preferably be no less than44.5 g, more preferably no less than 45.0 g, further preferably no lessthan 45.1 g, most desirably no less than 45.2 g.

The golf ball according to the present invention permits production withgood workability and exhibits excellent rebound resilience.

EXAMPLES

The invention will be described in more detail with reference to thefollowing examples and comparative examples, which are not intended torestrict the scope thereof.

Examples 1 to 5 and Comparative Examples 1 to 6

Polybutadiene shown in Table 1 was made into a rubber compound accordingto the formulation shown in Table 2. This rubber compound was formedinto a core of a two-piece golf ball by vulcanization at 155° C. for 20minutes. The core measures 38.9 mm in diameter and weighs 36.0 g. Thiscore was covered with a 1:1 mixture (by weight) of Himilan 1601 andHimilan 1557 by injection molding which forms dimples in the surface.After surface coating with paint, there was obtained a two-piece solidgolf ball measuring 42.7 mm in diameter and weighing 45.3 g.

The core was tested for deflection under a load of 100 kg (980 N) andrebound resilience in the following manner. The golf ball was examinedfor extrudability and flying performance in the following manner. Theresults are shown in Table 2.

Example 6 and Comparative Examples 7 and 8

A rubber compound was prepared according to the formulation shown inTable 3. This rubber compound was vulcanized at 155° C. for 20 minutesto make a core of a three-piece golf ball, which measures 36.4 mm indiameter and weighs 29.7 g. This core was enclosed with an intermediatelayer, 1.65 mm thick, which was injection-molded from a 35:35:30 mixture(by weight) of Surlyn 9945, Himilan 1605, and Dynalon 6100P.

The intermediate layer was further coated with a 1:1 mixture (by weight)of Pandex T8260 and Pandex T8295 by injection molding which formsdimples in the surface. Thus, there was obtained a three-piece solidgolf ball measuring 42.7 mm in diameter and weighing 45.5 g.

The core was tested for deflection under a load of 100 kg (980 N) andrebound resilience in the following manner. The golf ball was examinedfor extrudability and flying performance in the following manner. Theresults are shown in Table 3.

Example 7 and Comparative Examples 9 and 10

A rubber compound was prepared according to the formulation shown inTable 4. This rubber compound was vulcanized at 170° C. for 30 minutesto make a one-piece golf ball, which measures 42.7 mm in diameter andweighs 45.3 g. This golf ball examined for its performance. The resultsare shown in FIG. 4.

Deflection Under a Load of 100 kg

The solid core or the one-piece golf ball was tested for deflection (mm)under a load of 100 kg (980 N).

Rebound Resilience

Rebound resilience was evaluated by measuring the initial velocity withan apparatus of the same type as approved by USGA (United States GolfAssociation). In Examples 1 to 5 and Comparative Examples 1 to 6, theresults are expressed in terms of difference from the initial velocity(as the standard) in Comparative Example 1. In Example 6 and ComparativeExamples 7 and 8, the results are expressed in terms of difference fromthe initial velocity (as the standard) in Comparative Example 8. InExample 7 and Comparative Examples 9 and 10, the results are expressedin terms of difference from the initial velocity (as the standard) inComparative Example 10.

Flying Performance

Flying performance was evaluated by shooting the golf ball at a headspeed of 45 m/s (HS 45) with a driver (W#1, Tour Stage X500, loft 9°,shaft X, made by Bridgestone Sports) operated by a shooting machine.

Extrusion Workability

The extruded slug was examined for surface texture and shape, and theresult was rated according to the following criterion.

4: very smooth surface texture

3: slightly rough surface texture

2: fuzzy surface texture, extrudable

1: irregular shape, not extrudable

TABLE 1 Content Content of of Mooney Molecular cis-1,4 1,2-vinylviscosity weight bonds bonds (ML₁₊₄ distribution Kind Maker Catalyst (wt%) (wt %) (100° C.)) (Mw/Mn) BR BR 01 JSR Ni 96 2.5 46 4.2(polybutadiene) BR 11 Ni 96 2 43 4.4 BR 18 Ni 96 2 59 4.2 BR 730 Nd 961.3 55 3 CNB 700 Nd 96.2 1.3 43 2.8 BR 51 Nd 96 1.3 35.5 2.8

TABLE 2 Example Comparative Example 1 2 3 4 5 1 2 3 4 5 6 Core BR BR 01100 30 Formulation BR 11 (parts BR 18 70 by weight) BR 730 80 90 80 80  80 100 25 70 80 CNB 700 20 BR 51 20 10 20   20 75 30 20 (a) Amount 0.30.6 0.3 0   0.3 0.6 0.3 0.6 0.6 0.6 0.6 Perhexa Actual (0.12) (0.24)(0.12) (0)   (0.12) (0.24) (0.12) (0.24) (0.24) (0.24) (0.24) 3M-40 *¹(b) Percumyl D *² 0.3 0.6 0.3 1.2 0.3 0.6 0.3 0.6 0.6 0.6 0.6 Zinc oxide20 19.5 20 19.5  19 19.5 20 19.5 19.5 19.5 20 Antioxidant 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc acrylate 28 28 28 28   30 28 28 2828 28 28 Zinc salt of 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0pentachlorothiophenol Core Deflection under a 3.3 2.9 3.3 2.9 2.9 2.93.3 3.1 3.2 3.1 2.7 performance load of 100 kg (mm) Rebound resilience+0.4 +0.6 +0.4 +0.4  +0.8 +0.6 ±0 ±0 +0.1 −0.1 −0.2 (m/s) Extrusion 4 33 4   4 1 4 4 4 3 4 workability Flying #W1 Carry (m) 216.7 218.8 216.5217.0  220.5 218.5 212.5 212.1 213.3 211.0 210.3 performance HS 45 Total(m) 232.8 234.5 232.5 232.4  236.3 234.3 228.7 228.0 229.5 226.8 225.5Remarks: *¹ Half-life = 40 seconds, *² Half-life = 480 seconds. Perhexa3M-40 is a 40% diluted product. (The actual amount is in a parenthesis.)Note to Table 2 Comparative Example 1: The sample was good in reboundresilience but was very poor in workability because it was formed fromBR alone which was polymerized by using a Ni-based catalyst. ComparativeExample 2: The sample was good in workability but was poor in reboundresilience because it was formed from a rubber compound containing alarge amount of low-rebound BR which was polymerized by using a Ni-basedcatalyst. Comparative Example 3: The sample was poor in reboundresilience because it was formed from low-rebound BR which waspolymerized by using a Ni-based catalyst. Comparative Example 4: Thesample was poor in rebound resilience because it was formed from arubber compound containing a large amount of low-rebound BR which waspolymerized by using a Ni-based catalyst. Comparative Example 5: Thesample was poor in rebound resilience because it was formed from arubber compound containing a low-rebound BR which was polymerized byusing a Ni-based catalyst. Comparative Example 6: The sample was poor inrebound resilience because it was formed from a rubber compound whichwas not compounded with zinc salt of pentachlorothiophenol.

TABLE 3 Example Comparative Example 6 7 8 Core BR BR 01 100 FormulationBR 11 (parts by BR 18 weight) BR 730 80 100 CNB 700 BR 51 20 (a) Perhexa3M-40 Amount 0.3 0.6 0.6 (Half-life = 40 Actual (0.12) (0.24) (0.24)seconds) (b) Percumyl D 0.3 0.6 0.6 (Half-life = 480 seconds) Zinc oxide20 19.5 19.5 Antioxidant 0.1 0.1 0.1 Zinc acrylate 26 24 24 Zinc salt ofpentachlorothiophenol 0.6 0.6 0.6 Core Deflection under a load of 100 kg(mm) 3.9 3.9 4.1 performance Rebound resilience (m/s) +0.8 +0.6 ±0Extrusion workability 4 1 4 Flying #W1 Carry (m) 220.1 219.8 214.1performance HS 45 Total (m) 236.3 235.7 230.3 Note to Table 3Comparative Example 7: The sample was good in rebound resilience but waspoor in workability because it was formed from BR alone which waspolymerized by using a Ni-based catalyst. Comparative Example 8: Thesample was poor in rebound resilience because it was formed fromlow-rebound BR which was polymerized by using a Ni-based catalyst.

TABLE 4 Comparative Example Example 7 9 10 Core BR BR 01 100 FormulationBR 11 (parts by BR 18 weight) BR 730 80 100 CNB 700 BR 51 20 Percumyl D0.7 0.7 1 Zinc oxide 23 23 23 Antioxidant 0.2 0.2 0.2 Methacrylic acid23 23 23 Titanium oxide 1 1 1 Core Deflection under a load of 100 2.72.7 2.7 performance kg (mm) Rebound resilience (m/s) +0.8 ±0.8 ±0Extrusion workability 4 1 4 Flying #W1 Carry (m) 207.3 207.0 199.3performance HS 45 Total (m) 220.1 219.8 212.0 Note to Table 4Comparative Example 9: The sample was good in rebound resilience but waspoor in workability because it was formed from BR alone which waspolymerized by using a Ni-based catalyst. Comparative Example 10: Thesample was poor in rebound resilience because it was formed fromlow-rebound BR which was polymerized by using a Ni-based catalyst.

1. A golf ball which is formed from a vulcanized rubber compoundcomposed of, 100 parts by weight of a rubber base material, 10 to 60parts by weight of an unsaturated carboxylic acid and/or a metal saltthereof, 0.1 to 5 parts by weight of an organosulfur compound, 5 to 80parts by weight of an inorganic filler, and 0.1 to 3 parts by weight ofan organic peroxide, said rubber base material being composed of a firstpolybutadiene accounting for 80 to 95 wt % and a second polybutadieneaccounting for 5 to 20 wt % both polybutadiene being synthesized byusing a rare earth element-based catalyst, said first polybutadienecontaining no less than 60 wt % of cis-1,4 bonds and having a Mooneyviscosity (ML₁₊₄ (100° C.)) no lower than 50 and said secondpolybutadiene containing no less than 60 wt % of cis-1,4 bonds andhaving a Mooney viscosity (ML1+4 (1000C)) no higher than
 45. 2. The golfball of claim 1, wherein the first polybutadiene has a molecular weightdistribution Mw/Mn of 2.0 to 6.0, where Mw denotes a weight-averagemolecular weight and Mn denotes a number-average molecular weight. 3.The golf ball of claim 1, wherein the first polybutadiene is a modifiedpolybutadiene rubber which is obtained by synthesizing an ordinarypolybutadiene with a Nd-based catalyst and subsequently reacting thethus obtained polybutadiene with an end group modifier.
 4. The golf ballof claim 1, wherein the second polybutadiene has a molecular weightdistribution Mw/Mn of 2.0 to 6.0, where Mw denotes a weight-averagemolecular weight and Mn denotes a number-average molecular weight. 5.The golf ball of claim 1, wherein the second polybutadiene is a modifiedpolybutadiene rubber which is obtained by synthesizing an ordinarypolybutadiene with a Nd-based catalyst and subsequently reacting thethus obtained polybutadiene with an end group modifier.
 6. The golf ballof claim 1, wherein the second polybutadiene rubber has a Mooneyviscosity (ML1+4 (100° C.)) no higher than
 40. 7. The golf ball of claim1, wherein the organic peroxide is composed of two or more species oforganic peroxides which are specified by their half-life such that theratio of bt/at is 7 to 20, where at denotes the half-life of an organicperoxide (a) having the shortest half-life at 155° C. and bt denotes thehalf-life of an organic peroxide (b) having the longest half-life at155° C.
 8. The golf ball of claim 1, wherein the total content of theorganic peroxide is 0.1 to 0.8 part by weight for 100 parts by weight ofthe rubber base material.